You can get then over the internet, but shipping will bring the price up around $70/sack.

The best prices I've found were from breweries, but they can be hesitant to do this unless you are a friend of the brewery. You might be able to get a sack for close to $30 that way, if they sell near their cost.

The second best prices have come from group buys through clubs, which can sometimes manage to buy from suppliers like North Country Malt that won't sell to individuals. Then you should be able to get sacks for $40-50.